Media captionSyrian state TV footage appeared to show the chaos following the explosion

At least 13 people have been killed and many more injured by a powerful explosion in Syria's capital, Damascus, state media and activists say.

A bomb is believed to have been detonated in a square in the central district of Marjeh. Civilians and security personnel are among the dead.

Sporadic gunfire was heard in the area after the blast.

On Monday, Prime Minister Wael al-Halqi survived a car bomb attack, which had targeted his convoy in the capital.

The latest violence comes amid allegations that the Syrian authorities may have used chemical weapons against rebels fighting President Bashar al-Assad.

US President Barack Obama has said their use would be a "game changer" - and on Tuesday he insisted facts needed to be established before the US would rethink "the range of options available" for action.

But Syria's permanent representative to the UN, Bashar Jaafari, said his government was waiting for information about the dates and locations of the latest alleged uses of chemical weapons.

He told a news conference in New York that the Syrian government had told the UN that once it had investigated the alleged use of chemical weapons on 19 March in the Khan al-Assal district of the north-western province of Aleppo, it would look into more recent claims.

Damascus says rebel forces used chemical weapons at Khan al-Assal, but rebel commanders have accused government forces of carrying out the attack, citing reports of victims suffering breathing difficulties and bluish skin.

Widespread damage

Tuesday's bomb attack took place near a hotel, shopping centre and interior ministry building in Marjeh, a busy commercial district, the BBC's Jim Muir in Beirut reports.

Image caption
Dozens of civilians are reported to have been injured by the powerful explosion

It is not yet clear what the target of the attack was or who was behind it, our correspondent says.

However, it highlights that people in the centre of the city are no longer safe, he says.

State television showed footage of wounded people being carried away amid widespread damage. The area was shrouded in dense smoke. Several vehicles could be seen burning and many buildings had shattered windows.

The Syrian Observatory for Human Rights (SOHR) said the cause of the blast was a car bomb.

The UK-based activist group monitors human rights violations on both sides of the conflict via a network of contacts across Syria.

It estimates that March was the bloodiest month yet, with more than 6,000 people killed - a third of them civilians.

Image caption
The blast struck a busy commercial district of Damascus

Government forces and rebels have been fighting in and around Damascus for months, but neither have gained the upper hand.

Monday's blast targeting the prime minister was the latest bombing inside government-controlled areas of the capital.

The car bomb exploded as his convoy passed through the Mezzeh neighbourhood, reportedly killing a number of people, including Mr Halqi's bodyguard.

So far no group has said they carried out the latest bombings.

But they do not appear to carry the hallmarks of attacks linked to the Free Syrian Army or the jihadist al-Nusra Front, one of the most prominent rebel groups, our correspondent says.

Some opposition members accuse President Bashar al-Assad's government of planting the bombs in order to portray its struggle as one against terrorism, he adds.

The UN says more than 70,000 people have been killed since the uprising against Mr Assad erupted in March 2011.